The subject matter disclosed herein relates to a molded case circuit breaker having a current sensing unit.
Molded case circuit breakers (“MCCB”) are widely used to protect electrical lines and equipment, and are recognized by their rectangular plastic case. The MCCB monitors current through an electrical conductor and “trips” to open the electrical circuit and thus interrupt current flow through the circuit provided that certain predetermined criteria are met, such as an over-current condition. An electronic trip unit (“ETU”) is a device that can be used in conjunction with the MCCB to control the current (and/or voltage) versus time trip response. The ETU is a programmable device that measures and times current flowing through the circuit breaker and initiates a trip signal when appropriate.
Current-sensing transformers sense circuit current and provide current signals to the ETU for processing. Current transformers and Rogowski coils monitor the current in the MCCB. The current transformer is used for powering the ETU without use of external auxiliary power. The current transformer provides current output to the ETU that is proportional to the primary current flowing through it. The Rogowski coil is used for saturation free measurement of current in the conductor of the circuit breaker. The Rogowski coil provides a voltage output that is proportional to the time derivative of the current, rather than a current output like traditional current transformers. The current sensors, whether they are thermal-magnetic, thermal only, magnetic only, current transformers only, Rogowski coils only, or current transformers/Rogowski coils combination, are installed internal to the circuit breaker housing of the MCCB during assembly.